A catalyst has been used in many fields including various fields such as cleaning of various gases, in addition to the acceleration of a reaction such as a compound synthesis reaction and a fuel cell reaction. Many of traditionally used catalysts have a structure consisting of a porous body of a metal oxide such as alumina and silica or of carbon as a carrier, and an active metal, particularly a precious metal such as platinum, palladium and rhodium carried thereon.
In addition, recently, a multicomponent catalyst having a plurality of precious metals carried thereon for the purpose of improving activity has been commonly used. The alloying of the catalytic metal means to dissolve two or more active metals in each other, for instance, like platinum and palladium, or platinum and rhodium, and many of alloyed metals tend to show improved activity by interaction between constituent elements, which is a characteristic not seen in a catalyst having a single metal carried thereon. It is also expected that the alloying of the catalytic metals enhances interaction with the carrier and consequently shows the effect of inhibiting the deactivation of the catalyst.
Furthermore, on the carrier as well, effects of various carriers in addition to alumina, silica and carbon have been studied. For instance, in a field of a catalyst for treating an automotive exhaust gas, in order to smooth the atmosphere fluctuation of an exhaust gas atmosphere, a carrier added with ceria having oxygen occlusion discharge capability or with a ceria-zirconia solid solution having the oxygen occlusion discharge capability peculiar to the ceria and improved heat resistance has been recently used.
A conventional catalyst is normally produced by the steps of: impregnating a metal salt solution such as dinitrodiammineplatinum, chloroplatinic acid and rhodium nitrate in a porous oxide carrier; making an atomic metal carried on the carrier; and then calcining the carrier in a reducing atmosphere. In addition, a multicomponent catalyst is also produced by the steps of: preparing a metal salt solution containing a plurality of metals to be carried; mixing it with a carrier to make a plurality of metal ions adsorbed on the carrier; and drying and calcining it. The conventional catalyst produced by these production methods has a form in which the atomic metal particles are carried on the carrier.
[Patent Document 1]: Japanese Patent Application Laid-Open No. 60-50491
[Patent Document 2]: Japanese Patent Application Laid-Open No. 63-116741
This is because the activity of a catalyst is greatly affected by the state of carried catalyst particles when the catalyst carries the same metal and the same amount thereon, and the catalyst having a catalytic metal carried thereon in the state of clusters formed of gathering atoms shows higher catalytic activity than a catalyst having the catalytic metal dispersed and carried thereon in the state of atoms. The conventional catalyst is heat-treated so as to migrate atoms of the metal carried in an atomic state and cohere them into a catalyst particle having a functionable particle size. However, there are such problems as described below in the conventional catalyst.
The first problem of a conventional catalyst is a problem in controlling a particle size. When the catalyst is conventionally heat-treated, which is an indispensable step, it is not strictly taken into account how many atoms constitute a catalyst particle. Specifically, it is difficult to completely control a catalyst particle size of the conventional catalyst, so that the particle size of the catalyst particles is often ununiform. The catalyst having such ununiform catalyst particles shows inferior activity.
In addition, a conventional catalyst in a form of an alloy catalyst carries the alloy thereon often ununiformly distributed, and carries metals thereon often insufficiently alloyed. The alloyed metal requires that the atoms of constituent elements exist in the vicinity of each other atom, but in a catalyst having the atomic metal carried thereon in a conventional way, the metals are ununiformly distributed because of the difference of a zeta potential between them (difference of adsorptivity), so that the metal with high adsorptivity tends to be adsorbed on the surface of a carrier when heat-treated, whereas the metal with low adsorptivity is hardly adsorbed on the carrier surface and diffuses into the inner parts of pores in the carrier. Even if the carrier in such a state would be calcined, the metals with the high adsorptivity cohere with each other to form the catalyst particles on the carrier surface, whereas the metals with the low adsorptivity cohere with each other to form the catalyst particles in the inner parts of the carrier, so that the catalyst particle of a uniform alloy cannot be formed even though the carrier is thoroughly heat-treated. In addition, the insufficiently alloyed catalyst becomes inferior in activity, particularly initial activity right after having been produced, while being associated with the above described problem with the particle size of the catalytic metal particle.
A conventional catalyst is also required to improve the durability which is the capability of maintaining its activity for a long period. The durability constitutes a matter of concern in an application field used at a high temperature like a catalyst for cleaning an exhaust gas, and the cause of deterioration in activity is considered to be the migration and cohesion of catalyst particles caused by being heated during use. When the conventional catalyst is used in a high-temperature environment, the catalyst particles on a carrier become migratable by being released from the constraint by the carrier, and migrate, contact and cohere with the other catalyst particles to form coarse catalyst particles, so that the activity of the whole catalyst is deteriorated.
Furthermore, in a conventional catalyst, a state of catalyst particles tends to be affected by a type of a metal oxide of a carrier, so that the characteristics also tend to differ according to the type. For instance, in the case of the conventional catalyst which employs a metal oxide having comparatively low interaction to a metal, such as alumina, as the carrier, even though the catalyst carries metal atoms thereon, the metal atoms can easily migrate and cohere by subsequent heat treatment. However, in such a catalyst, the catalytic metal easily migrates even by heat onto the catalyst during use, forms coarse catalyst particles at a comparatively early stage, and loses its activity. On the other hand, in the case of the conventional catalyst which employs a metal oxide having comparatively strong interaction to a metal, such as ceria, as the carrier, catalytic metal atoms are locked in the carrier and become hardly migratable in a stage of having been carried by the carrier, and hardly cohere into an appropriate particle size even by heat treatment. As described above, the conventional catalyst acquires different characteristics according to the type of the carrier, and needs to select the carrier in accordance with use, which require a lot of complicated troubles in controlling products to a catalyst maker.
Under the circumstances, an object of the present invention is to provide a catalyst having catalyst particles formed on a carrier in an optimally carried state, so as to solve the above described problems of the conventional catalyst; and a production method therefor. Another object of the present invention is to provide a catalyst showing high activity and high durability not only in a comparatively-low-temperature environment but also in a high-temperature environment and to provide a production method therefor.